One of the key factors that affects the throughput of the fabrication of integrated circuits (IC) is the etch rate and/or deposition rate. In particular, as critical dimensions continue to decrease and respective aspect ratios of various features, such as self align contacts, etc., continue to increase, the ability to transport etch reactants and/or deposition materials to (and remove etch products from) the bottom of high aspect ratio (HAR) vias and contacts becomes inherently more difficult. This is primarily due to the lack of directionality of the neutral flow. For example, in etch applications, an improved method of transporting etch reactants to the bottom of high aspect ratio self align contacts (HAR SAC) is imperative for the continuance of the technology. A second key factor that contributes to the quality of the IC fabrication process is the process selectivity. For example, in a particular etch application, it is desirable to etch one identified material at a rate substantially greater than any other material present (such as the photo-resist mask, etc.). A third key factor that determines the yield and overall quality of an IC is the uniformity of the semiconductor fabrication processes (e.g., film etch and/or deposition), occurring at the surface of a substrate. In wafer processing systems, the rate, selectivity and uniformity of the deposition or removal of material is governed by the design of the overall reactor. A key element in this overall system design that can contribute to the aforementioned key factors for successful IC fabrication is the design of the gas delivery system and, in particular, the gas nozzles used to deliver gas to the interior of the plasma reactor chamber.
One system for processing ICs on semiconductor substrates typically includes a vacuum chamber, a pedestal for supporting the wafer in the chamber, a RF power generator for coupling RF power to a plasma within the vacuum chamber and a gas injection system for supplying gases to the chamber. If the reactor is an inductively coupled reactor, then it can include a coil antenna around the chamber connected to the plasma RF power source. Conversely, if the reactor is a capacitively coupled reactor, then it can include an additional parallel plate electrode facing the substrate that is connected to the plasma RF power source. Moreover, the wafer pedestal can also be connected to either the same or a separate RF power source. In other types of plasma reactors, there may be no coil antenna or opposite facing parallel electrode, and the plasma RF power source is connected solely to the wafer pedestal. Additional plasma sources may include an electron-cyclotron-resonance (ECR) source wherein microwave power is coupled to the plasma. In any case, the gas injection system of the reactor has one or more gas distribution apparatus. If multiple gas distribution apparatus are employed, each is typically disposed in a separate part of the reactor so as to provide gas to a different region within the chamber.
The gas distribution apparatus utilized depends on the particular requirements of the process being performed. In general, the gaseous specie(s) enter the vacuum chamber through a “showerhead” gas injection plate that comprises a plurality of small orifices (bores). The bores are typically constant area circular ducts that are typically 0.5 to 1 mm in diameter. A single injection plate may comprise several hundred to several thousand bores. Due to the effusive nature of the gas introduction, two distinct features of the flow through these bores include very low “bulk” velocities in a particular direction (i.e., gas molecules do not move collectively in a preferred direction at high velocities) and an overall lack of directivity. In essence, the gas “showers” down onto the substrate surface.
In order to achieve improved process uniformity, it is sometimes necessary to adjust the spatial distribution of the inlet mass flow and/or gas specie(s) to adjust the resultant neutral flow pressure field and flow dynamics in conjunction with other process parameters (i.e., RF field) to compensate for the inherent non-uniformity. In the prior art, most methods of adjusting the mass flow distribution typically fall into one of the following two categories: a) the adjustment of the spatial distribution of the bore area or the number density of the bore, hence, adjustment of A, or b) the adjustment of the bore mass flux or ρV. As stated above, the first method comprises a spatial distribution of the bore area or number density of bores. Several patents address the first method, including U.S. Pat. No. 4,780,169 and several patents filed within the Japanese Patent Office, including Japanese Patent Applications No. 2-198138, 6-204181, and 60-46029.
However, there are disadvantages to the first method. For example, a separate injection plate must be machined for each distribution tested, and it cannot be adjusted without breaking the vacuum or low pressure environment. With regard to the second method, U.S. Pat. No. 5,683,517 discloses a method of using a programmable gas flow divider to adjust the distribution of the mass flux to individual bores or clusters of bores. Other U.S. patents include U.S. Pat. Nos. 5,853,484 and 5,269,847. Each of these inventions includes adjustment of the mass flux to a plurality of sub-bores and all include the capability for in-situ adjustment of the mass flow distribution. However, the design can produce fairly complex and expensive plumbing arrangements for gas injection.
A second type of gas distribution apparatus comprises radial injection of the gas into the chamber from the reactor's sidewall, typically near the level of the wafer, during various processing operations (e.g., plasma enhanced chemical vapor deposition). This radial gas distribution apparatus may be used alone, or in combination with other gas distribution apparatuses, i.e., the so-called showerhead type of gas delivery nozzle mentioned above. In either of the two above described apparatuses, the gas injection lacks directivity, in particular, in a direction normal to the substrate surface. This inhibits neutral atom/molecule/radical deposition in deep, high aspect ratio trenches or vias when fabricating ICs.
One method of generating highly directive gas jets is to use properly designed gas nozzles to restrain the rate of gas expansion as a gas is expanded from a region of high pressure to a region of low pressure and accelerated towards the substrate. The prior art discloses gas nozzles for use in semiconductor tools such as plasma reactors. For example, U.S. Pat. No. 5,885,358 (the '358 patent) describes a gas injection system for injecting gases into a plasma reactor. The reactor has a vacuum chamber with a sidewall, a pedestal for holding a semiconductor wafer to be processed, and a RF power applicator for applying RF power into the chamber. The gas injection system includes at least one gas supply containing gas, a gas distribution apparatus having at least one slotted aperture facing the interior of the chamber, and one or more gas feed lines connecting the gas supply or supplies to the gas distribution apparatus. A preferred embodiment of a radial gas distribution apparatus is disposed in the chamber sidewall and includes plural gas distribution nozzles each with a slotted aperture facing an interior of the chamber. Gas feed lines are employed to respectively connect each gas distribution nozzle to separate ones of the gas supplies. However, a shortcoming of this system is that the gas is not optimally directed at the wafer surface, so as to enhance the statistical probability of gas atoms or molecules approaching the substrate at normal incidence to the substrate surface. Moreover, the system does not address the means by which the gas is introduced or expanded through the nozzle to achieve a directed gas flow nor does it even attempt to discuss the ability to tune the directivity of the gas injection.
U.S. Pat. No. 5,746,875 describes an invention that is embodied in a gas injection apparatus for injecting gases into a plasma reactor vacuum chamber having a chamber housing, a pedestal holding a workpiece to be processed, a device for applying RF energy into the chamber, the gas injection apparatus having a gas supply containing an etchant species in a gas, an opening in the chamber housing, a gas distribution apparatus disposed within the opening in the chamber housing which has at least one slotted aperture facing the interior of the chamber and a device for controlling the flow rate of gas from the one or more slotted apertures, and a gas feed line from the supply to the gas distribution apparatus. In a preferred embodiment, the gas distribution apparatus includes a center member surrounded by at least one annular member with a gap therebetween comprising the slotted aperture. Preferably, each of the members of the gas distribution apparatus comprises a material at least nearly impervious to attack from the etchant species. In one example, each of the members of the gas distribution apparatus comprises one of a ceramic, fused quartz, polymeric or anodized aluminum material and the gas feed line comprises stainless steel. Preferably, each of the members has its surface polished prior to assembly of the gas distribution apparatus. However, as before with the '358 patent, a shortcoming of this system is that the gas is not optimally directed at the wafer surface, so as to enhance the statistical probability of gas atoms or molecules approaching the substrate at normal incidence to the substrate surface. Moreover, the system does not address the means by which the gas is introduced or expanded through the nozzle to achieve a directed gas flow nor does it discuss the ability to tune the directivity of the gas injection.
U.S. Pat. No. 5,286,331 (the '331 patent) describes how in supersonic molecular beam etching, the reactivity of the etchant gas and substrate surface is improved by creating etchant gas molecules with high internal energies through chemical reactions of precursor molecules, forming clusters of etchant gas molecules in a reaction chamber, expanding the etchant gas molecules and clusters of etchant gas molecules through a nozzle into a vacuum, and directing the molecules and clusters of molecules onto a substrate. Translational energy of the molecules and clusters of molecules can be improved by seeding with inert gas molecules. The process provides improved controllability, surface purity, etch selectivity and anisotropy. Etchant molecules may also be expanded directly (without reaction in a chamber) to produce clusters whose translational energy can be increased through expansion with a seeding gas. However, the shortcomings of this system are several fold. First, the invention uses a single gas injection nozzle to expand a gas into an ultra-high vacuum (that ranges in chamber pressures of 10−8 to 10−14 Torr and less) to produce a supersonic molecular beam employed for neutral beam etching. Secondly, the design of the nozzle system includes a skimmer which would cause significant interference to the flow through the skimmer at chamber pressures above approximately 10 mTorr. Moreover, conventional pumping technology could not evacuate the chamber to the above-cited pressure for a number of nozzles necessary to produce a uniform process.
U.S. Pat. No. 5,108,535 describes a dry etching apparatus which includes a discharge room in which a gas plasma is created by a discharge, an ejection nozzle for ejecting the plasma gas, a first vacuum room into which the plasma gas is introduced through the ejecting nozzle by supersonic expansion of the plasma gas, and a second vacuum room including a skimmer for extracting a supersonic molecular flow, the supersonic molecular flow of the plasma gas taken into the second vacuum room being blown against the material to be etched. However, similar to the application described in the '331 patent, many of the same shortcomings are inherent to such a system designed for neutral beam etching.
To further understand the benefits of the design of the gas flow manifold of the present invention, it helps to understand the concept of choking for a continuum, isentropic gas flow and designing a nozzle unit to produce a supersonic gas jet. With reference to FIG. 1, there is shown a conventional Laval convergent-divergent nozzle 800 comprising an hour-glass cross-sectional shaped bore having a gas entrance region 810, a centrally located narrow throat region 820 and a gas exit region 830. Also, the gas entrance total pressure is Pt, the throat pressure is P*, the gas exit pressure is Pe and the plasma reactor chamber pressure in region 840 is Pc.
When nozzle 800 becomes choked, the Mach number M (the ratio of the local velocity to the speed of sound) is unity at nozzle throat 820. Once the flow of gas is sonic at the throat 820, it accelerates to supersonic speeds (M>1) when it experiences an increase in area (unlike a subsonic flow that decelerates during an area enlargement). Under such a condition, a nozzle with diverging walls after the throat accelerates the flow to supersonic speeds. Once the flow becomes supersonic, the flow characteristics (as defined by rays of pressure wave propagation) become real and are identifiable as Mach waves (expansion) and shock waves (compression). The directions of propagation of such waves are limited to a domain of influence, wherein a point within the entire domain can only affect the region that is downstream of that point and bounded by the left and right running characteristics that intersect at that point. Therefore, when M>1, pressure waves can not propagate back upstream through the nozzle and influence the incoming flow, i.e., volume flow rate or mass flow rate (when the gas entrance total pressure Pt is held fixed).
For a nozzle with a constant cross-sectional area (i.e., a straight cylindrical bore), the gas exit pressure Pe may be larger (even substantially larger, by several orders of magnitude) than the ambient chamber pressure Pc. In fact, when a divergent nozzle section is employed, it may produce either an under-expanded gas (i.e., one that has not entirely expanded to the chamber pressure) or an over-expanded gas (i.e., one that has expanded beyond the chamber pressure) condition. The latter condition generally results in a strong normal shock in the gas nozzle. Alternatively, in the under-expanded case, the gas exiting the bore freely expands in to the vacuum chamber. However, expansion waves reflecting from the wall adjacent to the exit of the bore opening coalesce to form a barrel shock which, in turn, creates a Mach disk a short distance downstream from the bore exit plane (depending upon the pressure ratio Pt/Pc it may be of order 10 nozzle diameters for a pressure ratio of order 100 to 200). Only by careful design of the area ratio and the nozzle contour can one achieve a pressure-matched condition comprising a collimated, uniform gas flow.
For most plasma reactor system applications, the low pressure environment of chamber interior region 840 of a plasma reactor chamber into which the gas is injected, is typically in the range between 1<Pc<1000 mTorr. Likewise, the gas entrance total pressure Pt is typically in the range between 0.1<Pt<100 Torr. Across these pressure ranges, the gas dynamics can change significantly due to the dependence of the Knudsen number (Kn) on the local pressure, and moreover, the resultant transition from a continuum flow to a free molecular flow (a consequence of the relatively large Kn). By definition, the Knudsen number is a non-dimensional parameter relating the mean-free path for gas atom (or molecule) collisions to a characteristic length scale for the flow. In the present invention, the appropriate length scale is the diameter of the nozzle, or alternatively, the axial length along which macroscopic properties of the flow vary significantly.
At the upper bound of the pressures previously mentioned (Pt>˜10−100 Torr), the gas flow through a nozzle having a bore diameter of approximately of the order 0.5 mm undergoes a sufficient number of collisions that the gas flow behaves as a continuum fluid; i.e., the mean free path of the atoms or molecules is much smaller than the characteristic flow length scales, or Kn<<1. Furthermore, the Knudsen number is sufficiently small (and the Reynolds number is sufficiently large) that a region of the continuum nozzle flow may be regarded as isentropic. During these conditions, the gas nozzle behaves similarly to the description provided above.
However, for pressure-matched conditions at gas exit region 830, for low-pressure applications, it is conceivable to observe transition flow effects due to the increase of Kn through nozzle 800. For example, as the gas expands with the area enlargement, the pressure decreases and Kn increases; i.e., the mean free path between gas atom (or gas molecule) collisions becomes large to the point that it becomes comparable to the nozzle characteristic length scale. In general, Kn will enter the transition regime (i.e., 0.01<Kn<1) and the gas may emanate from nozzle 800 as a free molecular flow. This phenomenon can be beneficial to the gas acceleration, since the mean free path at nozzle exit region 830 has become larger than scales across which shock waves may occur.
Midway through the pressure range mentioned above (0.5<Pt<5−10 Torr), viscosity plays a growing role in the gas flow through nozzle 800. Ultimately, there exists no region within the flow field that may be treated as isentropic, continuum fluid. Across approximately this range of pressure, the gas flow through the nozzle transitions to an effusive molecular flow. At lower pressures, gas/molecular collisions taking place within nozzle 800 will become more infrequent to the extent that the gas flow may exhibit the behavior of a free molecular flow (and no longer can macroscopic properties in the continuum sense adequately describe the behavior of the flow).
Accordingly, when higher mass flow rates are achieved, one can attain a higher source total pressure, i.e., a mass flow rate of 500 to 1000 sccm and greater. An advantage to operating under these conditions in semiconductor processing is that highly directive gas jets can be produced that may be organized to coalesce prior to impinging on a substrate being processed. Furthermore, the gas jets may be designed to transition to a free molecular flow at the gas exit region 830. In so doing, the expanding gas becomes a supersonic beam (that undergoes few collisions), with a direction predominantly normal to the substrate plane. Both experimental measurements and theoretical predictions (i.e., Direct Simulation Monte Carlo, DSMC) can be employed to analyze the transition of the continuum flow to the behavior of a free molecular flow.